Plate 02 of 13Grading and choosing
Aggregate Sizes, Grading and Choosing Material in General Terms
What clay, sand, gravel and cobble mean in millimetres, how a sieve test draws a gradation curve, and why grading changes how stone drains and packs down.
Two ideas sit behind almost every description of sand, gravel or crushed stone. Size says how big the pieces are. Grading says how many pieces of each size are present, and the two are easy to confuse. This page explains the particle-size terms that geologists and engineers use, with the millimetre boundaries, then shows how a sieve test turns a heap of stone into a gradation curve. It goes on to cover fines, particle shape, nominal maximum size, and how grading affects drainage and compaction, and it finishes with general guidance on matching material to a job. It is an educational overview. It names no product or supplier and it is not an engineering specification.
Particle-size terms and where the boundaries sit
Geologists have long split loose sediment into four size fractions: gravel, sand, silt and clay. The U.S. Geological Survey notes that these fractions follow the Wentworth grade scale of 1922. In that report, sand-sized particles run from just under 2 mm down to 62.5 micrometres, silt-sized particles from 62.5 down to 4 micrometres, and clay is anything smaller than 4 micrometres. Gravel-sized particles are the ones above 2 mm.
The Washington State Department of Ecology publishes a fuller version of the same scale in its sediment-data guidance, including the coarse end that matters for landscaping and construction stone. Because 1 mm equals 1,000 micrometres, its 0.0625 mm sand and silt boundary is the same line as the USGS figure of 62.5 micrometres.
| Class | Particle diameter | Sub-divisions listed |
|---|---|---|
| Boulder | Greater than 256 mm | None |
| Cobble | 64 to 256 mm | None |
| Gravel | 2 to 64 mm | Very coarse 32 to 64, coarse 16 to 32, medium 8 to 16, fine 4 to 8, very fine 2 to 4 |
| Sand | 0.0625 to 2 mm | Very coarse 1 to 2, coarse 0.5 to 1, medium 0.25 to 0.5, fine 0.125 to 0.25, very fine 0.0625 to 0.125 |
| Silt | 0.004 to 0.0625 mm | Four sub-bands, from very fine silt (0.004 to 0.008 mm) to coarse silt (0.031 to 0.0625 mm) |
| Clay | Less than 0.004 mm | Coarse, medium and fine clay |
Ecology's page is explicit that the sand and fines division is the one that matters most for many studies, and that fines mean silt and clay together. The same guidance points out that the sieve sets used by some ASTM methods are slightly offset from the Wentworth sieves, which moves the sand and fines cut from 0.0625 mm to 0.075 mm (the No. 200 sieve) unless a modified method is used. The FHWA documents cited below use the 0.075 mm figure, so the word "fines" can mean slightly different things in a geology paper and in a road specification. Everyday labels for aggregates vary from region to region as well, and a label alone does not tell you which sieve sizes it refers to.
Sieve analysis: how grading is measured
Grading is measured with a sieve analysis. A University of Memphis civil engineering course describes it simply: a dry sample is shaken through a stack of sieves with progressively smaller openings, and the mass left on each sieve is weighed. For particles larger than 0.075 mm the sieve method is used, while finer material is measured by letting it settle in water, a hydrometer test. The same course lists the openings of the U.S. standard sieves, for example 4.75 mm for the No. 4 sieve, 2.00 mm for No. 10, 0.425 mm for No. 40 and 0.075 mm for No. 200.
Coarse stone uses larger sieves. The California Department of Transportation's sieve-analysis method, California Test 202, separates coarse aggregate on sieves from 3 inch down through 2 1/2, 2, 1 1/2, 1, 3/4, 1/2 and 3/8 inch to the No. 4 sieve. It also shows why test conditions matter. The minimum sample grows with the stone size, from 1,000 g for a 3/8 inch nominal maximum size to 5,000 g for 3/4 inch, because a small scoop of large stone would not represent the pile. Other countries use metric sieve series and their own standards, so sieve names differ from place to place even where the idea is identical.
The result is reported as percent passing each sieve. A sample might show 100 percent passing a 1 inch sieve, 60 percent passing a 3/8 inch sieve and 4 percent passing the No. 200. Those numbers are the raw material for the gradation curve.
Reading a gradation curve
A gradation curve plots particle diameter on a logarithmic horizontal axis against percent finer on a vertical axis. The Memphis notes explain that the curve shows the range of sizes in a sample and also how those sizes are distributed. Three shapes cover most cases.
A uniform sample has most grains in one narrow size range, and its curve rises steeply. Engineers call this poorly graded, which sounds like a criticism but only describes the spread of sizes. A well-graded sample spreads across a wide range of sizes, giving a smooth, gradual curve. A gap-graded sample is a mix of two or more uniform fractions with sizes missing in between, so part of its curve runs flat. Open-graded material, a term used in road specifications, is closely related to the uniform case: it has few small particles, so large voids remain between the stones.
The Memphis course also gives numbers. D10 is the diameter at which 10 percent of the sample is finer, called the effective size, and D60 is the diameter at which 60 percent is finer. The uniformity coefficient compares D60 with D10. By that course's rule of thumb, a well-graded soil has a uniformity coefficient greater than about 4 for gravels and 6 for sands, and a coefficient of gradation between 1 and 3. Those thresholds come from soil mechanics teaching, and individual agencies write their own limits, so treat them as an illustration of the idea.
One trap for new readers is vocabulary. A geosciences text from SUNY Potsdam notes that geologists and engineers use opposite terms. Geologists describe a sample of near-equal grains as well sorted, while engineers call a sample with a wide range of sizes well graded, because it can be densely compacted. Which discipline is writing decides what the praise word means.
Fines, particle shape and nominal maximum size
Fines are the smallest particles in the mix. A Federal Highway Administration guide to gravel roads defines the very fine fraction as the part that passes a No. 200 sieve, particles smaller than 0.075 mm that cannot be seen individually by the human eye. An older FHWA report on base materials says that the fines in granular road base are limited for drainage and frost reasons to a maximum of 8 percent, with up to 12 percent permitted in sub-base. It adds that the same material packs densest at a fines content between 6 and 20 percent, yet its load-carrying capacity falls once fines exceed about 9 percent. Those figures describe highway layers in the United States and are not universal limits, but they show how a few percentage points of fines change behaviour.
Particle shape is the second variable. The same FHWA base report states that angular, nearly equidimensional aggregate with rough surface texture is preferred over rounded, smooth particles, and that thin, flat or elongated pieces are weaker and break down during compaction, creating more fines. The gravel-roads guide adds that crushed stone embeds in a surface better than rounded, natural-shaped stone, and that a rounded shape tends to shift under load.
Nominal maximum size is how a material is named by its largest stones without chasing the single biggest one. California Test 202 defines it as one sieve size larger than the first size to retain more than 10 percent. If the 1/2 inch sieve is the first to hold more than 10 percent of the sample, the nominal maximum size is 3/4 inch, the next sieve up. A lone oversized stone therefore does not change the label.
How size and grading change drainage and compaction
Grading pulls drainage and packing in opposite directions. The Michigan Department of Transportation's construction manual defines dense-graded aggregate as a mixture with an even distribution of different sizes, and open-graded aggregate as material with high porosity and void content that lets water drain freely. In a dense-graded mix, small particles fill the gaps between larger ones, so it compacts into a tight, strong layer but passes water slowly. In an open-graded mix, the gaps stay open, so water moves through quickly. The SUNY text makes the same point from the geology side: a uniform sample has the most porosity and so the best potential for fluid flow, while a poorly graded, uniform sample does not compact as well.
The FHWA base report says aggregate grading markedly influences base stability, drainage (permeability) and frost susceptibility, and that where a free-draining sub-base is required the fines are usually limited to 6 percent. The FHWA gravel-roads guide shows the cost of using the wrong blend. Fill with a high content of sand-sized particles drains well, it notes, but on a gravel road it stays loose and unstable. Base material has a very small percentage of clay or fines, and if it is used as a driving surface it will not form a crust to keep the material bound together. Surface gravel needs some plastic fines as a binder, with a plasticity index of at least 3 according to the guide's testing notes.
The details for two particular uses are covered on their own pages: gravel for driveways and drainage and crushed stone bases under paving and slabs.
Choosing material for a job in general terms
A sensible way to choose material starts with the job of the layer. A layer that carries load wants hard, angular, well-graded stone that locks together. A layer that moves water wants clean, uniform stone with very few fines. A surface that must stay bound wants some binder, and a bedding layer or fill has its own needs. Sands used for concrete, mortar or other purposes are described on the page about sand types and uses, and the role of aggregates inside concrete is covered in concrete and mortar ingredients. Material made from crushed concrete or asphalt is graded in the same way, as explained in recycled aggregates.
Written specifications exist because looking at a pile is not enough. The FHWA gravel-roads guide says plainly that "all gravels are not the same" and that real quality can only be determined by testing. For the same reason it recommends starting from a state specification where one exists. A specification names the sieves and the allowed range of percent passing for each, so two people can agree on what they are buying. Rules and customary practice differ between countries, regions and individual sites, and the local building authority, the road or highway agency, or a qualified engineer has the final word on what suits a particular job. For the basics of how material is produced before it is screened and graded, see what aggregates are and how they are made, and for the paperwork that arrives with a load, see reading a delivery ticket and measuring volume.
Frequently asked questions
What is the difference between gravel and crushed stone?
The FHWA gravel-roads guide describes gravel as sometimes loaded without processing, called bank run or pit run, with natural, often rounded stones. Crushing fractures a portion of the stone, and quarry gravels are composed of virtually all fractured particles. The fractured faces interlock better than smooth surfaces do.
Why do specifications list several sieve sizes instead of one?
Grading is a curve, and one number cannot describe it. The FHWA gravel-roads guide shows one state's example in which base course allows 3 to 12 percent passing the No. 200 sieve, while gravel surfacing allows 4 to 15 percent. The two materials differ at the fine end even though both are called gravel.
Can grading be judged by eye?
Only roughly. The FHWA guide says you can tell a little by looking at the material or running your hands through it, but that real quality is determined by testing, and it adds that fines below 0.075 mm cannot be seen individually.
Is fines content always a problem?
It depends on the job. The FHWA gravel-roads guide notes that a road surface needs fines for binding, whereas a drainage or base layer wants them kept low. Too many fines in a drainage layer can reduce how freely it passes water.
The short version
Size classes follow the Wentworth scale, with gravel from 2 to 64 mm and sand from 0.0625 to 2 mm, and a sieve analysis reports how much of a sample falls in each band. The shape of the gradation curve tells you whether the material is uniform, well graded or gap graded, and that shape largely decides whether it drains freely or packs tightly. Match the grading and shape to the job, and check the local specification and the advice of a qualified professional.